The invention relates to a measuring method and measuring system for measuring the imaging fidelity of an optical imaging system.
Optical imaging systems are employed in numerous fields of engineering and research that impose increasingly stringent demands on their imaging fidelity. An example of such a field is photolithographic fabrication of semiconductor devices and other types of microdevices, under which submicrometer-scale structures are created using high-performance projection lenses. Another example is photographic lenses of all types, which are usually subject to less stringent demands on their imaging fidelities.
Imaging optics frequently have elaborate layouts involving numerous lenses, which usually makes it impossible to derive their optical properties from theoretical computations. The optical properties of imaging systems thus must be reliably measured, where the accuracy of the method employed for testing the imaging systems involved is normally adapted to suit the demands imposed on their imaging accuracies.
Interferometric measurement methods are frequently employed. A device operating similarly to a shearing interferometer that allows making rapid, high-precision, measurements on photolithographic projection lenses is described in the German patent application having the filing code DE 101 09 929.0. In the case of that device, a mask illuminated by incoherent light is arranged in the object plane of the imaging system to be tested. The mask comprises a rigid, transparent, substrate fabricated from, for example, quartz glass, to which a two-dimensional object pattern is applied by, for example, suitable coating it with chromium. A reference pattern configured in the form of a diffraction grating is arranged in the image plane of the imaging system. Superimposing the waves created by diffraction on one another generates a superposition pattern in the form of an interferogram that is detected with the aid of a suitable (spatially resolving) detector.
Several interferograms having differing phases are required in order to compute two-dimensional phase distributions from these interferograms. Their phase may be varied either by displacing the diffraction grating on the object end of the imaging system involved or by displacing the mask on its object end. The lengths of travel employed in this procedure, which is termed “phase shifting,” are typically fractions of the grating periods involved. For practical reasons, in the case of interferometers employed for measuring high-resolution, microlithographic reduction lenses, the grating on the latter's image end bearing the reference pattern is usually translated, since both the lengths of travel on their object ends and the masses of the items that have to be translated are greater.
The accuracies of these phase shifts significantly affect measurement accuracy and must be accurately controlled to within a few nanometers in the case of applications in which spatial resolutions of the order of nanometers are to be assessed. Since a two-dimensional diffractive structure having several periodicity directions is preferably employed, displacing the grating substrate along mutually orthogonal periodicity directions orthogonal to the optical axis of the imaging system involved is required. In order to determine the contrast of interference fringes along an imaging direction, the contrast of interference fringes along another imaging direction is reduced to zero by a relatively rapid motion of the grating, with or without reversals of its direction of motion. In the case of this oscillatory motion of the grating in the plane of the grating, any displacements of the grating out of that plane are to be avoided at all times. These demands on the mechanism controlling the motions of the grating result in a relatively complicated design of that mechanism. Furthermore, reactions due to forces caused by accelerations may affect the entire setup and cause vibrations that will adversely affect metric accuracy.
Other interferometric devices for wavefront detection are described in, for example, the article entitled “Phase measuring Ronchi-test” by Omura, et al, that appeared in Applied Optics, Vol. 27, No. 3, pp. 523-528, German Patent Application DD 0 154 239, or German Patent Application DE 195 38 747.
Other testing methods, in particular, methods for measuring the distortion of optical systems, are based on utilization of the moiré effect. In the case of those methods, an object grating comprising, for example, a large number of parallel, opaque, lines forming an object pattern, is arranged in the object plane of the optical system to be tested. An image pattern similar to that object pattern is arranged in its image plane, where the object pattern and image pattern are adapted to suit one another such that a superposition pattern in the form of a moiré-fringe pattern is generated when the object pattern is imaged onto the image pattern by the imaging system. Imaging parameters, in particular, parameters indicating distortion generated by the imaging system, may be determined from the intensity distribution of this fringe pattern. Moiré methods are known from, for example, U.S. Pat. No. 5,767,959, whose content is largely identical to that of U.S. Pat. No. 5,973,773 or European Patent EP 0 418 054.
Separate light sources or illumination devices are provided for illuminating the respective semitransparent masks involved.